This invention relates to lead-calcium-tin-silver alloys for use in the positive grids for lead acid batteries. The alloy may be used to form thin grids by any method, including both expanded metal processing and book mold casting. Grids formed with the alloy harden rapidly, can be cured without resort to extraordinary measures and are stable and easily recyclable.
Modern automobile starting batteries require large numbers of thin grids. Proposed 42-volt battery systems will require even more and thinner grids. Sealed VRLA batteries for electric vehicle or hybrid electric service also require thin grids for rapid recharge. Thin positive grids also have utility in stationery batteries for uninterruptible power service or telecommunications service.
Production of thin grids whether conventional book mold cast, continuously cast, concast strip followed by expansion or direct continuous cast followed by rolling, results in a handling of the grid or the strip at high temperatures. The thinner the grid, the more difficult is the grid to handle at high temperatures. Production processes try to rapidly decrease the grid temperature with air, water, or water-cooled trim dies and platens depending on the process. The reduction in temperature is important for lead-calcium alloy grids because these are generally very weak at elevated temperatures and must be cooled to lower temperatures to prevent deformation or thickness change due to inadequate hardness. Despite rapid cooling to room temperature, many grid materials produced from low calcium alloys are extremely difficult to handle due to inadequate hardness at room temperature.
Thicker grids such as those of 0.060xe2x80x3 and above generally have more mass and are better able to be handled despite the low mechanical properties. Thus, thick grids can be cooled to room temperature more slowly than thinner grids. They may be able to be handled in pasting with lower hardness than thinner grids.
The mechanical properties of lead-calcium grid alloys are dependent not only on the temperature but also on the rate of aging after cooling to room temperature. The rate of aging is much more important in thin grids than thick grids.
During the past ten years, lead-calcium-based alloys have replaced lead-antimony alloys as the materials of choice for positive grids of both automobile and stationary lead-acid batteries. Lead-antimony alloys corrode more rapidly than lead-calcium alloys, antimony is released by grids during corrosion, and during the recharge process antimony is transferred to the negative plate where it causes unacceptable loss of water from the electrolyte, particularly in areas of high heat. Lead-calcium alloys do not suffer the water loss during service and, thus, can be processed into grids for maintenance or sealed lead-acid batteries.
Lead-calcium alloys have a very low freezing range and are capable of being processed into positive and negative grids by a variety of grid manufacturing processes, such as conventional book mold casting, rolling and expanding, continuously casting followed by expansion or punching, continuous grid casting, and continuous grid casting followed by rolling. The continuous grid manufacturing processes decrease battery grid and plate production costs.
About ten years ago, the automobile manufacturers modified the exterior of the vehicles to make them more aerodynamic. This design change caused considerably less air to flow through the engine compartment, considerably increasing the underhood temperature.
At that time, lead-calcium alloys were used that generally contained relatively high calcium content (0.08% or higher) and relatively low tin content (0.35-0.5%). Positive grids produced from these alloys hardened rapidly and could be handled and pasted into plates easily. The addition of aluminum to the lead calcium alloys and the method of manufacturing these alloys dramatically reduced calcium oxide generation during processing and permitted production of grids with much better control of the calcium content.
These alloys contained Pb3Ca. The higher underhood heat environment leads to increased corrosion of the positive grids in these alloys due to the presence of this Pb3Ca in the alloy and failure of the batteries due to corrosion and growth of the positive grids. New lead-calcium alloys were developed to address these problems. They are described in U.S. Pat. Nos. 5,298,350, 5,434,025, 5,691,087, 5,834,141, 5,874,186, as well as DE 2,758,940. These alloys contain much lower calcium than previous alloys because lower calcium produces lower corrosion rates.
Silver has been added to lead and lead alloys for many years to reduce the corrosion of the lead alloy when used as an anode or positive grid of a battery. Rao et al. in U.S. Pat. No. 4,456,579, Nijhawan in U.S. Pat. No. 3,990,893, and Geiss in U.S. Pat. No. 4,092,462 describe lead-antimony alloys for battery grids containing silver as an additive to reduce grid corrosion. The lead-calcium alloys referred to above also contain silver, which further reduces the rate of corrosion, and contain sufficient tin to react with virtually all the calcium to form stable Sn3Ca. The grids produced from the lead-calcium-tin-silver alloys have very high resistance to corrosion and growth of the positive grids during testing and in vehicle use, particularly at elevated temperatures.
Rao describes a lead-calcium-tin-silver alloy for positive automobile battery grids in U.S. Pat. No. 5,298,350 which contains 0.025-0.06% calcium, 0.3-0.7% tin, 0.015-0.045% silver, and may contain 0.008-0.012% aluminum. Further refinements of the alloy for direct cast strip are taught in Rao et al. in U.S. Pat. No. 5,434,025 where the calcium range is reduced to 0.02-0.05%, the tin content reduced to 0.3-0.5%, and the silver range increased to 0.02-0.05%. This patent also teaches utilizing strontium or mixed calcium/strontium as a replacement for the calcium. Rao et al. also teach in U.S. Pat. No. 5,691,087 the use of lead-calcium-tin-silver alloys for positive plates of sealed batteries with a composition of 0.025-0.06% calcium, 0.3-0.9% tin, and 0.015-0.045% silver. Rao et al. further refine the lead-calcium-tin-silver alloys for positive grids using the same calcium content ranges described above, but with higher tin contents and a lower level for the silver content based on the methods of grid production. In U.S. Pat. No. 5,874,186, Rao et al. teach an alloy having 0.03-0.05% calcium, 0.65-1.25% tin and 0.018-0.030% silver.
Anderson et al. in U.S. Pat. No. 5,834,141 describe a wider calcium range 0.035-0.085%, higher tin content 1.2-1.55%, and lower silver content 0.002-0.035% range than the patents of Rao and Rao et al. According to Anderson et al., the composition must be varied depending on the method of grid manufacture. If the alloy is to be book mold cast, the alloy must include aluminum and have 0.035-0.055% calcium, 1.2-1.55% tin, 0.025-0.035% silver and 0.005% aluminum. In contrast, a grid formed by the expanded metal process must contain 0.045-0.085% calcium, 1.2-1.55% tin and 0.002-0.0049% silver.
Larsen describes a method of producing directly cast strip of at least 0.060xe2x80x3 thickness from lead-calcium-tin-silver alloys in U.S. Pat. No. 5,948,566. Larsen""s alloy contains 0.01-0.06% calcium, 0.03-1.0% tin, 0.01-0.06% silver and optionally 0.003-0.01% aluminum. Assmann describes similar alloys in German patent DE 2758940 with a calcium content of 0.02-0.1%, a tin content of 0.44-1.90%, and a silver content of 0.02-0.1%. Yasuda et al in U.S. Pat. No. 4,939,051 describes the use of a foil of lead-silver-tin pressure bonded to a rolled sheet for a grid production process by expansion. Reif et al. in U.S. Pat. No. 4,725,404 describes the use of copper and/or sulfur to modify the grain structure of lead-calcium (tin) alloys. Finally, Knauer in U.S. Pat. No. 6,114,067 describes a lead alloy containing about 0.06-0.08% calcium, 0.3-0.6% tin, 0.01-0.04% silver and 0.01-0.04% copper which strengthens relatively quickly and can be used in batteries.
The grids produced from these alloys, however, are not without problems. The very low calcium contents (0.02-0.05%) generally utilized in the grid alloys produce grids which are very soft, difficult to handle, and harden very slowly. To utilize grids produced from these alloys, the cast material must be stored at room temperature for long periods of time or artificially aged at elevated temperatures to bring the material to sufficiently high mechanical properties to be handled in a pasting or expander/paster machine. On the other hand, calcium levels above 0.082% can result in formation of Pb3Ca rather than Sn3Ca.
Battery grids produced from the lower calcium/high silver-tin alloys are extremely corrosion-resistant. However, in order to be made into a battery plate, a grid must be pasted with a mixture of leady lead oxide, sulfuric acid, water and some additives. After pasting, the plates are cured to permit the paste (active material of the battery) to firmly attach itself to the battery grid. This permits good electrical contact between the grid and the active material.
During curing the grids are corroded to permit the paste to adhere to the grid. Battery manufacturers must now go to great lengths to corrode the very corrosion-resistant grids. These include treating the grids for long periods of time in hot steam environments to produce a corrosion film on the grid surface; treating the surface of the grids with alkaline reagents, peroxides, or persulfates; or long curing times at high temperature and humidity for as long as five days. In every production method, the failure mechanism of the battery is now generally active material disengagement from the positive grid rather than positive grid corrosion.
An additional problem for the above alloys is the relatively low tin content of 0.3-0.6%. Low tin contents permit the formation of non-conductive oxide layers between the grid and active material when the battery becomes discharged. The resistance of these oxide products may prevent adequate charge acceptance during recharge of the battery if it becomes discharged, thus resulting in premature failure.
The silver added to these alloys enters the recycled lead stream when the batteries are recycled. While some silver may be utilized by recycling back into the battery grid alloys, the silver may need to be removed to lower levels in the lead used for the active material of the batteries, particularly for sealed service. This leads to additional costs for battery recycling.
It is the object of this invention to provide a lead alloy which can be utilized in the production of the positive grids of lead acid batteries which can withstand the high temperature corrosion of the underhood environment of the automobile.
Another object is to produce thin grids by any method desired (continuously cast-expansion or punched, roll-expansion, continuously cast, continuously cast-rolled, or conventional book mold casting) using an improved alloy which will harden rapidly so that the grid can be utilized in a short period of time after production without excessively long aging time or artificial aging.
A further object of the invention is to increase the ability of paste to adhere to the grid surface during curing so that no extraordinary measures such as steaming or long curing times are required to produce a good grid/active material bond.
Still another object of the invention is to improve the charge acceptance of the battery produced with the improved grid alloy so that it can be adequately recharged if the battery becomes discharged.
Another object of the invention is to permit recycling of the batteries produced using the alloy more easily and at lower costs by reducing the silver content.
A further object is to increase the creep resistance and mechanical properties of the battery grid alloys so that the grids can better resist the effects of elevated temperatures without additional silver.
Other advantages of the grids are the improved stability of the grain structure resulting in reduced corrosion and the improved retention of the mechanical properties and active material at elevated temperatures.
This invention provides a lead acid battery grid produced from a lead alloy which contains calcium in an amount above 0.060 and below 0.082%, tin above 1.0 and below 1.2%, silver between 0.005 and 0.020%. The alloy may optionally further contain between 0.002 and 0.030% aluminum. In an alternative embodiment, the alloy contains between 0.005% and 0.05% copper in place of some of the silver, provided, however, the silver is never less than 0.005%.